1. Technical Field
The present invention relates generally to a production method of a semiconductor module which is equipped with a resin-molded package of a power semiconductor chip and a heat spreader working to spread heat from the power semiconductor chip and which may be of a 1-in-1 structure into which a single power semiconductor chip(s) such as an IGBT or a power MOSFET for use as either of an upper arm (i.e., a high side device) or a lower arm (i.e., a low side device) of an inverter is resin-molded, or a 2-in-1 structure into which two power semiconductor chips for use as the upper and lower arms, respectively, are resin-molded.
2. Background Art
One of typical semiconductor modules is equipped with a resin mold package in which a semiconductor chip(s) and a heat spreader(s) serving to dissipate heat, as generated by the semiconductor chip, are disposed. As the resin material for such mold, thermosetting resin such as epoxy resin is usually used in order to improve thermal endurance of the package.
The thermosetting resin is, however, difficult to break down after being hardened. Therefore, when any part of the thermosetting resin has broken down, the expensive semiconductor chip must also be scrapped.
In recent years, a demand for reusing the semiconductor chips has been increasing with increasing awareness of environmental issues.
Japanese Patent First Publication No. 2006-165534 teaches a semiconductor module which consists of stacked resin molded packages and coolant paths. In each of the resin molded packages, a power semiconductor chip(s) and heat spreaders are embedded. When one of the resin molded packages has become defective, the semiconductor module may be disassembled to remove only the defective package and reused by replacing it with a new one.
However, when the resin molded package is removed from the semiconductor module, the surfaces of the packages are susceptible to damage. It is undesirable to replace such a resin molded package itself. Such damage causes deterioration of quality of the mold and results in need for the package to be scrapped even though the power semiconductor chip which is embedded in the package operates properly.
The semiconductor module, as taught in the above publication, may be reused by replacing one of the resin molded packages which has been broken down with a new one. However, when the power semiconductor chip of one of the resin molded packages is operating properly, but its resinous mold is scratched during disassembling of the stack of the resin molded packages, the need usually arises for replacing the resin molded package, That rein molded package will, however, be removed even though the power semiconductor chip thereof operates properly. This is undesirable in terms of efficiency in utilization of limited resources.
In order to alleviate the above problem, the inventors of this application have devoted themselves to studying the structure of the semiconductor module which enables the power semiconductor chip to be reused when the surface of any one of the resin molded packages is scratched. Specifically, the resin molded package is made up of a thermosetting resin-made body and a thermoplastic resin-made shell. The thermosetting rein-made body has components such as the power semiconductor chip, etc. embedded therein. The thermoplastic resin-made shell is wrapped around an outer circumference of the thermosetting resin-made body and has a coolant passage formed therein. This structure enables one of the rein molded packages to be detached from the semiconductor module if it is scratched and heated to soften and remove the thermoplastic resin-made shell, thereby taking only the thermosetting resin-made body in which the power semiconductor chip is disposed out of the resin molded package. The rebuilding of the resin molded package may be achieved by placing the thermosetting resin-made body within a molding tool and injecting thermoplastic resin into the molding tool to form the thermoplastic resin-made shell around the thermosetting resin-made body again.
This enables the power semiconductor chips to be reused when the surface of the resin molded package is scratched. This is very useful for the case where one of the resin molded packages is scratched during disassembling of the semiconductor module to take out another of the resin molded packages which has become defective.
We have however, found that the above structure of the resin molded packages made up of two resin molds: the thermosetting resin-made body and the thermoplastic resin-made shell faces the following problem.
The formation of the thermoplastic resin-made shell after the thermosetting resin-made body is made results in a lack of adhesion between them, which may cause air gaps to appear at the interface between the thermoplastic resin-made shell and the thermosetting resin-made body, thus leading to leakage of the coolant to the power semiconductor chip in the thermosetting resin-made body.
FIGS. 10(a) and 10(b) demonstrate an example of such leakage of coolant within a semiconductor module made up of a stack of resin molded packages 10.
Each of the resin molded packages 10 is equipped with a positive electric terminal 15 connecting with a collector of an IGBT, negative electric terminal 16 connecting with an emitter of the IGBT, and signal terminals 17 for use in sensing given parameters. The positive electric terminal 15, the negative electric terminal 16, and the signal terminals 17 are partially exposed outside the resin molded package 10. When an air gap occurs between the thermosetting resin-made body 21 and thermoplastic resin-made shell 22, it may cause, as indicated by solid lines in FIG. 10(b), the coolant to leak into the air gap from a coolant passage and flow out of the resin molded package 10 to the exposed portions of the leads 15 and 16 and the signal terminals 17. This may, as indicated by solid lines in FIG. 10(a), result in electrical leakage between the positive electric terminal 15 and the negative electric terminal 16 or among the signal terminals 17.